High pressure water injection pump system

ABSTRACT

The invention provides a high pressure water injection pump system, comprising at least two water injection pump units connected in series, each pump unit comprising a motor in a motor compartment operatively connected to drive a pump in a pump compartment, distinctive in that at least the pump units following the first pump comprises a choke compartment arranged between the pump and motor compartments, a leakage seal arranged between the pump and choke compartment, the choke compartment having an outlet; and the motor compartment of at least the pump units following the first pump unit is dimensioned according to a lower pressure class than the operatively connected pump compartment.

FIELD OF THE INVENTION

The present invention relates to water injection into subsea underground reservoirs, such as for stimulating production from a reservoir containing petroleum.

BACKGROUND OF THE INVENTION AND PRIOR ART

Water injection in order to stimulate production from a petroleum reservoir takes place by pumping water at high pressure down injection wells. The high pressure water is pumped into the reservoir or formation in fluid communication with the reservoir. Thereby the reservoir pressure can be maintained and petroleum can migrate toward production wells.

In order to achieve sufficient pressure from subsea pumps, a number of pumps can be arranged in series. However, the pressure increases for each pump unit and subsequent pump units are exposed to the full pressure of upstream pump stages, the pressure increases for each pump stage or pump unit. Each pump stage or pump unit comprises a motor and a pump coupled together inside a housing.

A typical consideration for design, operation and maintenance is to have one type of pump unit in all stages. The pressure housing must be chosen so that the last pump stage or the maximum pressure to which the pumps are exposed govern the choice. Accordingly, all pump units may have the same pressure housing, resulting in heavy and expensive pump units but simplifications with respect to spare part storage and maintenance.

Alternatively the pressure housing class increases for each pump stage, but this has the effect that the pump units of lower pressure housing rating can not replace pump units of higher pressure housing ratings.

The effect is either that the pump system in total can weight many more tons than required and cost millions in excess, or millions in excess must be spent on additional spare parts. A demand exists for a pump system without the above-mentioned disadvantages.

SUMMARY OF THE INVENTION

The demand is met with the present invention.

More specifically, the invention provides a high pressure water injection pump system, comprising at least two water injection pump units connected in series, each pump unit comprising a motor in a motor compartment operatively connected to drive a pump in a pump compartment, distinctive in that at least the pump units following the first pump comprises a choke compartment arranged between the pump and motor compartments, a leakage seal arranged between the pump and choke compartment, the choke compartment having an outlet; and the motor compartment of at least the pump units following the first pump unit is dimensioned according to a lower pressure class than the operatively connected pump compartment.

The term high pressure water injection system means that at least two water injection pumps must be arranged in series in order to achieve the desired pressure.

For the pump units following the first pump unit, the motor compartment pressure housings are dimensioned for equal pressure as the pressure housing of the first pump. More preferably, all motor compartment pressure housings are dimensioned for equal pressure class. The effect is that the pressure housing or pressure shell of the pump compartments are thinner and lighter, and also associated equipment such as penetrators, sensors and flanges can be dimensioned for a lower differential pressure.

For the pumps following the first pump unit, or for all pump units, the motor compartment pressure housings are preferably dimensioned for equal pressure as the pressure housing of the first pump (depending on the maximum supply pressure to the motor barrier fluid system, this can be higher than the pump suction pressure). If the inlet pressure to the first pump is ambient seawater pressure, or slightly below (depending on piping and any potential water treatment facility upstream of the injection pumps), i.e. the inlet is raw or treated seawater, an equal overpressure for motor compartment pressure housing class is preferably chosen for all motor compartment housings. Said overpressure will typical be 10-20 bar overpressure in operation, but the pressure rating of the motor compartment should equal the maximum supply pressure of the barrier fluid system, which will be typically about 50-100 bar above the pressure of a choke chamber, which will be described below.

A leakage seal is arranged between the pump compartment housing and a choke compartment, the choke compartment is for each pump unit following the first pump unit arranged between the pump and motor compartments and each choke compartment is connected to a leakage line connected to the inlet line to the first pump. More preferably a leakage seal is arranged between the pump compartment housing and a choke compartment, the choke compartment is for each pump unit arranged between the pump and motor compartments and each choke compartment is connected to a leakage line connected to the inlet line to the first pump. For some embodiments, preferably embodiments using raw seawater or treated seawater as injection water, the leakage line is omitted and the flow leaked out to the choke compartment is let out to the sea.

More specifically, each choke compartment includes a choke seal in a partition wall toward the pump compartment, the choke seal is a labyrinth seal or any type of mechanical seal having a choke or leakage effect, such as a spring loaded mechanical seal of which the spring adjusts the leakage opening, preferably a controlled or adjustable choke effect, and a more fluid tight mechanical seal (very low leakage, typically a few litres per day in normal operation) is arranged in the partition wall between each choke compartment and motor compartment. The fluid tight seal between the choke compartment and motor housing is preferably connected to a barrier fluid system at a pressure equal to or higher than the pressure of the choke compartment. The barrier fluid pressure in the motor is controlled by a pressure volume regulator (PVR).

In a preferable embodiment seawater is used as the motor cooling medium, the seawater is provided by at least one separate seawater cooling pump arranged to pump seawater through the motor compartments. Even more preferably, injection water is used as the motor cooling medium, provided by the injection water for cooling leaking through a choke or leakage seal arranged in the partition wall between the pump and motor compartments, said seal can be a labyrinth seal or any type of mechanical seal having a choke or leakage effect such as a spring loaded seal, preferably the seal leakage rate is proportional to the motor cooling demand. A fully insulated motor is crucial for reliability of the embodiments using seawater or injection water as cooling medium. Insulation materials like cross linked polyethylene XLPE, or PEEK, or others, can be used.

For the embodiments pumping in seawater for use as coolant in the motor, the seawater motor impellers are preferably arranged to the motor shaft.

Typical pump compartment pressure classes are 5000 psi (345 bar), 10000 psi (690 bar), 150000 psi (1035 bar) and 20000 psi (1380 bar). Typical pressure classes of the motor compartment will be significantly lower, for example 100 bar or less.

The invention also provides a method for arranging a high pressure water injection pump system, comprising at least two water injection pump units connected in series, each pump unit comprising a motor in a motor compartment operatively connected to drive a pump in a pump compartment. The method is distinctive by arranging for at least the pump units following the first pump a choke compartment between the pump and motor compartments, a leakage seal between the pump and choke compartment, and an an outlet from the choke compartments; and dimensioning the motor compartment of at least the pump units following the first pump unit according to a lower pressure class than the operatively connected pump compartment, more preferably the motor compartment pressure housings of the pump units following the first pump unit are dimensioned for equal pressure as the pressure housing of the first pump, most preferably all motor compartment pressure housings are dimensioned for equal pressure class.

In addition the invention provides use of the high pressure water injection pump system of the invention, for injecting water at high pressure into a subsea underground reservoir.

FIGURES

The invention is illustrated with seven figures, of which

FIGS. 1 a and 1 b illustrate embodiment of a pump system according to the invention, with not-identical pump units,

FIGS. 2 a and 2 b illustrate embodiments of a pump system according to the invention, with identical pump units, and

FIGS. 3 a, 3 b, 3 c and 3 d illustrate embodiments of a pump system of the invention using seawater and/or injection water as cooling medium.

DETAILED DESCRIPTION

Reference is made to FIG. 1, illustrating an embodiment of a pump system according to the invention for which the first pump unit is different to the second and subsequent pump units. More specifically, the first pump unit has no choke compartment and hence no choke seal between a pump compartment and choke compartment and no leakage line connected to a choke compartment. This has advantage of no loss due to leakage in the first pump and thereby a somewhat better efficiency. Also a shorter and thereby stiffer shaft can be used.

To the contrary, this embodiment has disadvantage in that the operator has two different versions of pumps and must have additional spare parts and maintenance procedures. An alternative embodiment, but with leakage to sea and hence no leakage line, is illustrated in FIG. 1 b.

Reference is the made to FIG. 2 a illustrating an embodiment of a pump system according to the invention for which all pump units can be identical. For this embodiment a choke compartment is provided between the pump compartment and motor compartment for all pump units, a leakage line is arranged from each choke compartment to the inlet of the first stage and a choke seal is arranged on the shaft between each pump compartment and choke compartment. Accordingly, versatility and a fewer spare parts can be allowable but a slight loss in efficiency due to the leakage flow over a choke seal also for the first stage pump will be the result. An alternative embodiment, but with leakage to sea and hence no leakage line, is illustrated in FIG. 2 b.

The leakage over the choke seal and the leakage line are dimensioned in order to avoid overpressure in the choke compartment. The leakage line dimensioning must take into account that the maximum flow rate is proportional to the square of the flow velocity. However, a relief valve opening at an intended pressure can be arranged in the choke compartment.

Another embodiment is to use the pumped medium as coolant flow for the motor. In this configuration, the leakage flow can be routed from the pump through the motor and either discharged to sea (FIG. 3 c), or routed back to the inlet of the first pump to recover the pumped fluid (FIG. 3 d). The advantage with this embodiment is that the injection pump will itself generate the required differential pressure across the leakage seal between the pump and the motor compartment. The leakage seal can then be designed such that a sufficient flow rate is achieved to cool the motor without use of any additional circulation impeller for the motor cooling fluid. As pump/motor speed increases, the injection pump will generate an increasing differential pressure across the leakage seal, and hence flow rate, to sufficiently cool the increasing motor losses at higher rotational speeds. This means that the typical flow versus pressure characteristic of the leakage seal between the pump and motor compartment can be designed to provide the sufficient cooling flow through the motor at any speed.

Accordingly, the invention provides a high pressure water injection pump system, comprising at least two water injection pump units connected in series, each pump unit comprising a motor in a motor compartment operatively connected to drive a pump in a pump compartment, distinctive in that it comprises a leakage seal arranged between the pump and motor compartments, the motor compartment having an outlet and being cooled by the leaked in water and optionally by introduced seawater, and preferably the motor compartment of at least the pump units following the first pump unit is dimensioned according to a lower pressure class than the operatively connected pump compartment.

In alternative embodiments, the motor could also be cooled using a cooling pump that draws seawater from the surroundings (raw seawater or slightly treated seawater) or from the same or a different subsea water treatment system and discharged back to the inlet line (FIG. 3 a) or discharged to sea (FIG. 3 b).

Which one of the embodiments illustrated will be preferred in each case can be estimated using good engineering and cost calculation practice. The embodiments using the pumped fluid, the injection water, as coolant, will often be the most preferred embodiment since no barrier fluid system is required and the cooling can be adjusted to the cooling demand since the seal leakage rate can be designed to meet the motor cooling demand. However, each of the embodiments will result in significant simplifications and savings compared to prior art high pressure water injection pump systems, since the pressure class of parts of the system can be significantly reduced, with resulting savings in weight, cost, spare parts stock for the pumps and associated equipment such as penetrators. The cost saving can be counted in millions Norwegian Kroner, the weight savings can be counted in metric tons. 

1. A high pressure water injection pump system, comprising: at least two water injection pump units connected in series, each pump unit comprising: a motor in a motor compartment operatively connected to drive a pump in a pump compartment; wherein at least the pump units following a first pump of the at least two water injection pumps comprises: a choke compartment arranged between the pump compartment and the motor compartments; a leakage seal arranged between the pump compartment and the choke compartment, the choke compartment having an outlet; and wherein the motor compartment of at least the pump units following the first pump unit is dimensioned according to a lower pressure class than an operatively connected pump compartment.
 2. The high pressure water injection pump system according to claim 1, wherein for the pump units following the first pump unit, the motor compartment pressure housings are dimensioned for equal pressure as the pressure housing of the first pump.
 3. The high pressure water injection pump system according to claim 1, wherein all motor compartment pressure housings are dimensioned for equal pressure class.
 4. The high pressure water injection pump system according to claim 1, wherein each choke compartment is connected to a leakage line connected to the inlet line to the first pump.
 5. The high pressure water injection pump system according to claim 1, wherein a leakage seal is arranged between the pump compartment housing and a choke compartment, the choke compartment is for each pump unit arranged between the pump and motor compartments and each choke compartment is connected to a leakage line connected to the inlet line to the first pump.
 6. The high pressure water injection pump system according to claim 4, wherein each choke compartment includes a choke seal in the partition wall toward the pump compartment, the choke seal is a labyrinth seal or any type of mechanical seal having a choke or leakage effect, such as a spring loaded mechanical seal of which the spring adjusts the leakage opening, preferably a controlled or adjustable choke effect, and a fluid tight mechanical seal is arranged in the partition wall between each choke compartment and motor compartment.
 7. The high pressure water injection pump system according to claim 1, wherein seawater is used as a motor cooling medium, the seawater is provided by at least one separate seawater cooling pump arranged to pump the seawater through the motor compartments, preferably the seawater pump is arranged as seawater pump impellers on the motor shaft.
 8. The high pressure water injection pump system according to claim 1, wherein injection water is used as a motor cooling medium, provided by the injection water for cooling leaking through a choke or leakage seal arranged in a partition wall between the pump and motor compartments, said seal can be a labyrinth seal or any type of mechanical seal having a choke or leakage effect such as a spring loaded seal, preferably the seal leakage rate is proportional to the motor cooling demand.
 9. A method for arranging a high pressure water injection pump system, the pump system comprising: at least two water injection pump units connected in series, each pump unit comprising: a motor in a motor compartment operatively connected to drive a pump in a pump compartment, the method comprising: arranging for at least the pump units following the first pump a choke compartment between the pump and motor compartments, a leakage seal between the pump and choke compartment, and an an outlet from the choke compartments; and dimensioning the motor compartment of at least the pump units following the first pump unit according to a lower pressure class than the operatively connected pump compartment, more preferably the motor compartment pressure housings of the pump units following the first pump unit are dimensioned for equal pressure as the pressure housing of the first pump, most preferably all motor compartment pressure housings are dimensioned for equal pressure class.
 10. Use of the high pressure water injection pump system according to claim 1, for injecting water at high pressure into a subsea underground reservoir.
 11. A high pressure water injection pump system, comprising: at least two water injection pump units connected in series, each pump unit comprising: a motor in a motor compartment operatively connected to drive a pump in a pump compartment, a leakage seal arranged between the pump and motor compartments, the motor compartment having an outlet and being cooled by the leaked in water and optionally by introduced seawater, and preferably the motor compartment of at least the pump units following the first pump unit is dimensioned according to a lower pressure class than the operatively connected pump compartment. 